Modern magnetically based electronic article surveillance systems generally derive their parentage from 1934 French Patent No. 763,681. That patent depicts the use of markers formed of a piece of low coercive force, high permeability alloy, such as permalloy, and teaches that when the magnetization of such a piece is reversed by a magnetic field alternating at a fundamental frequency, detectable harmonics of that frequency will be produced. More recently, various investigators have developed magnetic markers which have dual-status capabilities. Typically, as disclosed in U.S. Pat. Nos. 3,665,449 (Elder et al.) and 3,747,086 (Peterson), such dual status markers include at least one piece of remanently magnetizable material which when magnetized has associated therewith a magnetic field which biases the low coercive force, high permeability material so as to alter the signal produced when the biased material is in the interrogation field. Systems utilizing such markers are designed so that when the remanently magnetizable material is unmagnetized, the low coercive force material is free to produce certain harmonics on which detection is based. In that state, the marker is then regarded as being sensitized. Alternatively, when the remanently magnetizable material is magnetized, the resultant magnetic bias on the low coercive force piece prevents the formation of the same harmonic response such that the marker is not detected, and the magnetized marker is regarded as being desensitized. Systems operating in such a manner have become quite commercially successful, particularly in circulating libraries and the like for preventing the theft of books. In such installations, a marker is inconspicuously secured within the book to be protected. The magnetizable piece is remotely magnetized in order to allow the book to be checked out and is subsequently demagnetized when the book is checked in. As knowledge of such a procedure has become more commonplace, potential thieves have been known to carry a small permanent magnet in attempts to magnetize, i.e., desensitize the markers to thereby thwart detection as the book is carried through the interrogation zone. The use of such systems may be limited in retail stores and the like where markers may not be concealed within the protected article and are more accessible to such unauthorized desensitization, and where more valuable merchandise warrants a higher degree of protection.
Furthermore, the system disclosed by Elder et al. ('449) utilizes a marker containing a very elongated piece of high permeability material. The reversal of the magnetization in such a piece by an interrogation field alternating at a fundamental frequency results in the production of a characteristic response containing very high order harmonics of the fundamental frequency. Unless the piece has such an elongated shaped, signals containing readily detectable very high order harmonics will not be produced.
As noted above, most magnetic EAS systems operate in a magnetize to desensitize mode. U.S. Pat. No. 3,983,552 (Bakeman et al.) depicts an alternative magnetic EAS system which also uses a dual status marker. In that system, magnetization of a remanently magnetizable "keeper" element causes even order harmonics to be produced, upon which detection in the system is based. While the markers are thus sensitized when magnetized, the marker and system there depicted is not known to have been commercially practiced.